1. Field of the Invention
The present invention relates to an optical head device that applies light beam for recording/reproducing information on/from an optical disc that is used as a recording medium.
2. Description of the Related Art
An optical disc apparatus includes at least an optical head device that has an objective lens and a photo detector. By the focusing light beam applied to the recording area of a recording medium, that is, an optical disc via this optical head device, information recorded on the optical disc are read out. In addition, information are recorded on an optical disc by the focusing light beam applied.
Recently, with the increase of information to be recorded, improvement of data recording/reading speed on/from optical discs has been desired.
In view of the above, a separate type optical head device which is composed of a semiconductor laser and a main body of the optical head device, that is, a fixed portion for driving the semiconductor laser, and a movable portion including an objective lens that is used for applying light beam to an optical disc and taking reflecting light out of the optical disc, both of the fixed and movable portions are independently arranged.
On this separate type optical head device, the movable portion that is composed of an objective lens and a mirror (or a mirror prism) is constructed in small size and light weight. Further, the fixed portion and the movable portion are optically connected by a galvano mirror of which reflecting angle is variable.
However, on this separate type optical head device comprising the fixed portion and the movable portion optically connected by the galvano mirror, there is such a problem that an offset signal is generated in a tracking signal that is returned to the fixed portion by a slight tilting of the galvano mirror, so called deflection in proportion to the deflection of the galvano mirror.
In this case, reproduction of information is not accurately carried out but becomes unstable because the center of laser beam applied to a recording medium is shifted from the center of tracks formed on the recording medium. This will become a defect in increasing the recording density.
Further, on an optical disc apparatus the quantity of laser beam that is output from a semiconductor laser is generally controlled, For this control of quantity of light, a half mirror is provided on the way of the optical path for leading the laser beam generated from the semiconductor laser to an optical disc in order to lead a part of laser beam to the outside of the optical path. The laser beam led to the outside of the optical path is applied to a photo detector. Quantity of the laser beam applied to this photo detector is measured and the measuring signal is fed back to the semiconductor laser to control the laser beam quantity emitted from the semiconductor laser to an optimum level.
However, in the method to control the quantity of laser beam emitted from the semiconductor laser as described above, a photo detector has been arranged at the outside of the optical path to receive the laser beam led to the outside of the optical path by the half mirror provided on the way of the optical path. The photo detector arranged at the outside of the optical path will result in the optical head device becoming large in size.